We hired a Unity dev long before Henshin was brought on (3 years prior). We brought the dev in to try and fix elements of Fungus and create it to be more of what we needed. We started on the save system and to bring it off the registry. That dev, tried but it kept bricking the whole project, he even took it to others to try and find a solution and we were left with nothing. We spent upwards of $20k trying to get that done.
I do agree with the statement regarding divorcing ourselves from Fungus earlier. But I want to build on your assumptions a little. We're in year 9, with June 15th being our 10 year anniversary. Fungus was operating just fine through both the cells and bordello fill outs (that is 5 yrs of updates). We talked about rebuilding SU from the ground up many times in Unity, and the problem is always this. If development is on-going (since we can't just pause) we're going to be behind the Fungus version for an exceedingly long time, because the fungus version keeps getting added to. There is a risks to that such as: what if the dev drops out, or we run into platform issues. We then would have wasted YEARS and a lot of funds on a project that doesn't get released.
For comparision Henshin rebuilt SU for Android (it's not even caught up) in 2 years behind the scenes (we never announced development). Our estimates for him to catch up to active SU would have been another 1.5 years or more. Almost 4 years of full time work. To us, it is a risk we will not take with our supporters money.
The solution is breaking apart SU to be more download, device, and operating system friendly. We can get these events rebuilt in Ren'py in a couple months, adding content is faster than Fungus (we're talking days), and bug fixes happen within 12 hours of testing. The last Gotham update was bug tested and fixed within 4 hours, whereas Fungus SR7 could spend a week tracing flowcharts.